The use of non-dispersive infrared spectroscopy to detect hydrocarbon gases is well established. It essentially involves transmitting infrared radiation along a path in an area being monitored; the wavelength of the infrared radiation is chosen so that it is absorbed by the gas of interest (hereafter called the “target gas”) but not substantially absorbed by other gases in the atmosphere of the area being monitored. If monitoring out-of-doors, the wavelength should ideally not be absorbed by liquid or gaseous water (e.g. in the form of humidity, condensation, fog, rain or spray). The intensity of the radiation that has passed along the path in the area being monitored is measured and the attenuation in the intensity of the radiation gives a measure of the amount of target gas in the monitored area.
However, factors other than absorption by the target gas also attenuate the infrared radiation, including obscuration of the detecting beam, atmospheric scattering of the radiation, contamination of the surfaces, e.g. by dirt or condensation, and ageing of components. The reliability of infrared gas detectors is significantly improved by the use of a reference; such a reference is usually infrared radiation at a different wavelength which ideally is a wavelength at which the target gas does not exhibit significant absorption. Radiation at more than one reference wavelength may be used; likewise more than one sample wavelength may be used. The ratio between the signal obtained at the wavelength(s) where the target gas does absorb (the “sample” wavelength(s)) and the signal obtained at the wavelength(s) where the target gas does not significantly absorb (the “reference” wavelength(s)) compensates for the attenuation caused by environmental conditions since ideally the signal at the reference wavelength(s) and the signal at the sample wavelength(s) will both be affected to the same extent by effects (other than the presence of target gas) that attenuate the radiation.
It is known to monitor the presence of toxic gases in an atmosphere using point gas detectors, which can be electrochemical or optical (the term “toxic” gas in the context of the present specification means a gas or vapour other than oxygen and nitrogen, such as hydrogen sulphide, hydrogen fluoride, ammonia, sulphur dioxide, carbon dioxide and carbon monoxide). The provision of point gas detectors gives rise to problems when monitoring a large area since the placing of numerous detectors throughout the area is expensive. Furthermore, if the build up of target gas takes place between detectors, it will not be detected. Open path gas detectors with a path length in excess of 1 meter, typically at least 10 m, allow a much larger area to be monitored by a single instrument.
The use of open path gas detectors has been made more attractive by the ready availability at a reasonable price of tuneable diode lasers, which can be tuned to a very narrow wavelength to detect characteristic absorbency wavelengths of target toxic gases. However, the levels of toxic gas that must be detected are low, typically 5 ppm (parts per million) and can be lower, e.g. 1 ppm. At such low levels, the noise in the detector can be greater than the signal of the target gas, making it very difficult to detect such low levels of toxic target gases. In addition, the signal can become indiscernible due to drift in electronic or optical components over time, variations in temperature and/or atmospheric conditions, etc. In addition, the use of coherent laser irradiation from a tuneable laser diode can give rise to interference fringes where the variation in the intensity of the radiation between the bright and dark fringes far exceeds the signal arising from the presence of low levels of the target gas.
Accordingly, no low cost reliable open path gas detector for toxic gases measuring target gas levels as low as 10 ppm has hitherto been possible.
GB-2353591 describes an open path gas detector that uses a tuneable laser diode as the radiation source directing a beam across a measuring path to a radiation detector in order to detect target gas within the path. The laser diode transmits radiation in a very narrow line width, much narrower than the absorption peak of a target gas. In such a known system, the wavelength of the laser diode is scanned across the absorption band of the target gas with a frequency f; the absorption band of the target gas is shown by line B in FIG. 1. In the process of scanning, the intensity of the transmitted laser radiation also varies with a frequency f; a graph of the variation of intensity with wavelength is shown as plot A in FIG. 1. The radiation transmitted is sensed by a detector that produces a signal proportional to the intensity of the radiation incident on it. A plot of intensity against time is not shown but is sinusoidal. If the atmosphere contains no target gas, the variation of the intensity of the radiation is given by plot A of FIG. 1 and the signal from the detector has a frequency that is the same as the scan frequency f. However, if there is target gas in the atmosphere, it will absorb the radiation, thereby attenuating the radiation reaching the detector. The resulting plot of the intensity of the radiation detected is a combination of curves A and B, as shown in FIG. 2. As will be appreciated, a plot of intensity against time has an additional frequency component of 2f.
The greater the amplitude of the 2f component, the greater amount of target gas there is in the measuring path. The 2f component (and higher harmonic components) of the signal can be determined using a phase-sensitive measuring amplifier (lock-in amplifier). The effect of the target gas on the 1f component will be relatively small compared to the 2f component. Consequently, a quotient formed from the 2f component and the 1f component can give a measure of the amount of target gas in the measuring path. The 1f and 2f components will be influenced in a similar manner to numerous attenuation conditions, for example the length of the measuring path, obscuration of the detecting beam, atmospheric scattering etc. Therefore, the 2f:1f quotient provides a measure of the amount of target gas in the measuring path.
Various elaborations on this basic technique are also known, for example it is possible to vary the median wavelength of the laser diode output at a slow frequency as compared to f. This provides a number of 2f:1f quotients, which can be analysed mathematically to provide a more reliable measure of the concentration of the target gas.
To obtain the variation in wavelength necessary to scan across the gas absorption band of a target gas, the electrical current through the laser is varied and consequently the optical output power also varies. Due to the nature of laser diodes, the magnitude of the 1f component is necessarily large. The magnitude of the 2f component is a function of the gas absorption and will be small for low levels of toxic gas. The 2f:1f quotients are therefore very small, typically 10−4 to 10−6 and the small value of this quotient is a substantial disadvantage of this technique since it is difficult to measure accurately.
Electronic assemblies employed to drive the laser and implement the phase sensitive measuring amplifier cause harmonic distortion of the signals. As the 1f component of the signal propagates through these electronic assemblies, any non-linear characteristics will result in harmonics of the 1f component being generated, including a component at 2f. This additional 2f component is summed with the 2f component resulting from absorption by the target gas leading to incorrect measured target gas concentrations, which can also give rise to false alarms, leading, in some cases, to a lack of credibility in the equipment.
In GB-2353591, the median value of the scanned wavelength is controlled by a feed-back circuit, as follows. A beam splitter is provided in the laser diode beam and part of the beam is directed along the measuring path and part is directed at a detector; a cell that holds a sample of the target gas (or some other substance having a suitable known absorption characteristic) is placed in front of the detection unit and so absorbs radiation at the wavelength of the target gas. The signal from the detector will show whether or not the wavelength of radiation emitted by the diode scans the absorption band of the target gas by determining the 2f:1f quotients for this feed-back beam in the same way as for the measuring beam, as discussed above. If the wavelength of the laser diode has drifted, this will be evident from the signal from the detector and allows a correction to be applied to the laser diode to bring it back to the correct wavelength.
One disadvantage of the above arrangement is that the beam splitter provides interference fringes that can swamp the signal of the target gas when it is present at a low concentration in the measuring path, as discussed above.
It is often difficult to provide optimum alignment of the measuring path between the transmitter unit and the detector unit at opposite ends of the measuring path. GB-2353591 suggests two-way communication link between the detector unit and the transmitter unit. The transmitter unit includes steering mirrors for changing the direction of the transmitted beam; the transmitted beam is periodically scanned and the optimum direction of the beam is determined as that at which the intensity measured by the detecting unit is greatest; the communication link between the detector unit and the transmitter unit provides feedback on the optimum position of the steering mirrors to achieve alignment.
One problem with open path gas detectors is water condensation on the optics, which obscures the transmitted beam. Accordingly, the optics are maintained at a temperature above the dew point to prevent such condensation. However, the heating of the optics adds to the complication of the system and it consumes substantial quantities of energy.